Early Descriptions of the Main Crater Morphology

Johann Wilhelm Ludwig von Luce (1756-1842) is usually acknowledged as the discoverer of the Kaali craters. From the description of the main crater he compiled and published in 1827 (Luce 1827) appears that he had mapped the lake as early as 1780. However, it is obvious that islanders were aware of the existence of the exotic hollow much earlier. The morphology of the crater and uplifted dolomitic rocks gave rise to several legends and tales.

It was believed that the lake had no bottom and its waters were hiding an entrance to the hell. This belief was partially triggered by statements of some lake researchers. For instance, Luce wrote in his work of 1827: "The water in the lake is always clear and fresh and it is very deep. When I fifty years ago exerting all my strength threw an about-4-m-long pole into the lake, and it flew out of the water in a few seconds, and I grabbed it, I couldn't find any sign suggesting that the tip of the pole had touched the

* In the scientific literature the name Reinwald is written in different ways: Reinwald, Reinwaldt, Reinvaldt, Reinvald. In this paper the first variant is used.

Fig. 1. Location of the Kaali impact craters and Piila Bog in the central part of the Island of Saaremaa. Numbers on the map indicate the location of small craters.

mud on the lake bottom". In reality, the water depth in the lake is 5 to 6 metres during high-water periods. During low-water periods the basin is almost dry (Fig. 2). The most rapid lake-level rise was recorded in 1927 when, as a result of heavy rainfall that lasted from 6 to13 September, the water rose by nearly 2 m (Lougas 1996).

Hofman was the first to describe the structure of the Kaali main crater in 1837. He noted that the Kaali depression bore an astonishing resemblance to maars - volcanic funnels on the Eifel plateau - and the tilt of the

Fig.2. Lake Kaali in the main crater during the low stand of water on October 10, 2000. A peat sample from a depth of 1.18-1.25 m in the dighole in the northeastern side of the crater (the right bank on the photo) was dated as 2958±51 BP. Photo by T. Moora.

uplifted dolomite layers on the crater slopes indicated an explosion the force of which had been directed from bottom upwards (Hofman 1837). In 1839, during an archaeological expedition on the Island of Saaremaa, Kruse, professor of general history at Tartu University, compiled a schematic plan of Lake Kaali, showing also the location of park roads and surrounding buildings. The plan was published three years later (Kruse 1842).

Hofman's theory suggesting an abrupt eruption of water, steam, gas and mud in the Kaali crater area was supported by Wangenheim von Quelen (1849, a.o.), who presented a visual plan and assumable cross-section of the Kaali main crater. Linstow (1919), a supporter of gas eruption, presented also a scheme of the main crater, four smaller craters and the surroundings of the crater. As a source of gas eruption he considered the organic-rich Dictyonema argillite rocks well known in northern Estonia. In the area under consideration, their bedding depth is about 200 m.

In the autumn of 1927, following the order of the Mining Department of the Ministry of Trade and Industry of Estonia, Reinwald explored the solid bed in the craters by digging and boring (Reinwald 1928). One borehole was put down at the base of the main crater from outside to a depth of 63.14 m, and two smaller ones were sunk inside the wall, the form and character of the lakebed were explored and water-level variations were studied. Reinwald established that the bottom of the crater is approximately horizontal and has a somewhat curved form. A funnel-shaped depression with a prolonged form was discovered in the hard bottom. Ground magnetic measurements (Aaloe et al. 1976) and several borings (Saarse et al. 1991) showed that the main crater is a bit asymmetric, being about 0.4 m deeper at the northwestern margin than in the central part. As another peculiarity of the crater, Reinwald mentioned huge dolomitic blocks tilted upwards from inside at an angle of 30-40 degrees (Fig. 3). After clearing the slope on the north side of the crater, it became evident that the general thickness of the tilted dolomites was 8 m, and they were absolutely identical with the upper part of the dolomitic strata at the exterior base of the crater where the aforementioned borehole had been put down. Beneath these tilted dolomite blocks, white or slightly brownish powder with soft pieces of whitish dolomite was discovered. In the centre of the bottom large stones had been heaped up for a monument to a deceased owner of the estate. The stones originated partly from the interior of the crater itself, and partly from elsewhere.

Based on the above facts, Reinwald reached the conclusion that the craters were of meteoritic origin (Reinwald 1928). The same hypothesis had already been advanced in 1919 by Kalkun-Kaljuvee (in Kaljuvee 1933). In September 1927, Meyer and Kraus from Riga visited the Kaali

area under the guidance of Reinwald. They were accompanied by Alfred Lothar Wegener (1880-1930), founder of the theory of continental drift. As a result of the short five-day field work Kraus, Meyer and Wegener (Kraus et al. 1928) more or less simultaneously with Reinwald persisted the idea of meteoritic origin of the Kaali craters. The same opinion was expressed by Hinks (1933), Fisher (1936, 1938) and Kranz (1937), but the last doubt about the genesis of the Kaali craters disappeared only in 1937 when Reinwald (1938) found meteoritic pieces containing nickel and iron (8.5 and 91.5%, respectively). Thus, at that time the Kaali craters were the second terrestrial object in the world - after the Diablo/Arizona crater in the USA- which could have been generated by extraterrestrial forces. With the decision of the Nature Conservation Council of Estonia the Kaali craters were taken under nature protection in November 1937.